Google Script to send out volunteer notifications using this spreadsheet: https://docs.google.com/spreadsheets/d/1nw1z3AXUhXaB-zY-e-nBvGdA0P4a4LRzhY6f5_pQ8HA/edit#gid=0

ShiftSender.gs

function sendEmails() {
  //Get the active spreadsheet file (the one you've opened)
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  
  //Get the active sheet, and the cells you've highlighted in it
  var dataSheet = ss.getActiveSheet();
  var dataSelection = ss.getSelection();
  var dataRange = dataSelection.getActiveRange();
  
  //Get the template text from the sheet named "Copy". This script expects...
  var templateSheet = ss.getSheetByName("Copy");
  var emailTemplate = templateSheet.getRange('B3').getValue(); // the email text in B3
  var subjectTemplate = templateSheet.getRange('B2').getValue(); // the subject line in B2
  var senderName = templateSheet.getRange('B4').getValue(); // the sender name in B4
  var bcc = templateSheet.getRange('B5').getValue(); //the BCC address in B5
  
  //use the Google-provided getRowsData function to take each row of data and break it up in to columns, named with the column header
  var objects = getRowsData(dataSheet, dataRange,2); // the "2" here tells the function that the column header is in row 2.
  
  //for each row of data (each volunteer selected), do the following: 
  for (var i = 0; i < objects.length; i++){
    var rowData = objects[i];
    
    var emailText = fillInTemplateFromObject(emailTemplate, rowData); //Merge in the data for the email text
    var emailSubject = fillInTemplateFromObject(subjectTemplate, rowData); //Merge in the data for the subject line
    
    //Pack up the data in to the format that Google needs to send by GMail
    var email = {
      'to': rowData.email, 
      'subject': emailSubject,
      'body': emailText,
      'name': senderName
      }
    if(bcc){
      email.bcc = bcc;
    }
    
    //send the email!
    MailApp.sendEmail(email);
  }
}

// Adapted from the Google version just to have more accurate variable names  
function fillInTemplateFromObject(template, data){
  // start with the template
  var copy = template;
  
  //find all the mail-merge target in the template
  var templateVars = template.match(/\$\{\"[^\"]+\"\}/g);
  
  //for every mail-merge target found:
  for (var i = 0; i < templateVars.length; ++i){
    //set the value of the mail-merge target from the data
    var variableData = data[normalizeHeader(templateVars[i])];
    
    //update the template with the data for the target
    copy = copy.replace(templateVars[i], variableData || '' );
  }
  
  //send the personalized copy back
  return copy;
}

//////////////////////////////////////////////////////////////////////////////////////////
//
// The code below is reused from the 'Reading Spreadsheet data using JavaScript Objects'
// tutorial. - https://developers.google.com/apps-script/articles/mail_merge
//
//////////////////////////////////////////////////////////////////////////////////////////

// getRowsData iterates row by row in the input range and returns an array of objects.
// Each object contains all the data for a given row, indexed by its normalized column name.
// Arguments:
//   - sheet: the sheet object that contains the data to be processed
//   - range: the exact range of cells where the data is stored
//   - columnHeadersRowIndex: specifies the row number where the column names are stored.
//       This argument is optional and it defaults to the row immediately above range;
// Returns an Array of objects.
function getRowsData(sheet, range, columnHeadersRowIndex) {
  columnHeadersRowIndex = columnHeadersRowIndex || range.getRowIndex() - 1;
  var numColumns = range.getEndColumn() - range.getColumn() + 1;
  var headersRange = sheet.getRange(columnHeadersRowIndex, range.getColumn(), 1, numColumns);
  var headers = headersRange.getValues()[0];
  return getObjects(range.getValues(), normalizeHeaders(headers));
}

// For every row of data in data, generates an object that contains the data. Names of
// object fields are defined in keys.
// Arguments:
//   - data: JavaScript 2d array
//   - keys: Array of Strings that define the property names for the objects to create
function getObjects(data, keys) {
  var objects = [];
  for (var i = 0; i < data.length; ++i) {
    var object = {};
    var hasData = false;
    for (var j = 0; j < data[i].length; ++j) {
      var cellData = data[i][j];
      if (isCellEmpty(cellData)) {
        continue;
      }
      object[keys[j]] = cellData;
      hasData = true;
    }
    if (hasData) {
      objects.push(object);
    }
  }
  return objects;
}

// Returns an Array of normalized Strings.
// Arguments:
//   - headers: Array of Strings to normalize
function normalizeHeaders(headers) {
  var keys = [];
  for (var i = 0; i < headers.length; ++i) {
    var key = normalizeHeader(headers[i]);
    if (key.length > 0) {
      keys.push(key);
    }
  }
  return keys;
}

// Normalizes a string, by removing all alphanumeric characters and using mixed case
// to separate words. The output will always start with a lower case letter.
// This function is designed to produce JavaScript object property names.
// Arguments:
//   - header: string to normalize
// Examples:
//   "First Name" -> "firstName"
//   "Market Cap (millions) -> "marketCapMillions
//   "1 number at the beginning is ignored" -> "numberAtTheBeginningIsIgnored"
function normalizeHeader(header) {
  var key = "";
  var upperCase = false;
  for (var i = 0; i < header.length; ++i) {
    var letter = header[i];
    if (letter == " " && key.length > 0) {
      upperCase = true;
      continue;
    }
    if (!isAlnum(letter)) {
      continue;
    }
    if (key.length == 0 && isDigit(letter)) {
      continue; // first character must be a letter
    }
    if (upperCase) {
      upperCase = false;
      key += letter.toUpperCase();
    } else {
      key += letter.toLowerCase();
    }
  }
  return key;
}

// Returns true if the cell where cellData was read from is empty.
// Arguments:
//   - cellData: string
function isCellEmpty(cellData) {
  return typeof(cellData) == "string" && cellData == "";
}

// Returns true if the character char is alphabetical, false otherwise.
function isAlnum(char) {
  return char >= 'A' && char <= 'Z' ||
    char >= 'a' && char <= 'z' ||
    isDigit(char);
}

// Returns true if the character char is a digit, false otherwise.
function isDigit(char) {
  return char >= '0' && char <= '9';
}